1. Field of the Invention
The invention relates to an apparatus for initial charging of the output capacitors of a three-point pulsed rectifier system, and to a method and apparatus for controlling it.
2. Description of Related Art
According to the prior art, series resistors are inserted into the supply lines for initial charging of the output capacitors of pulsed rectifier systems, in particular in the circuit described in EP 94 120 245.9-1242 (A 2612/93), and are bridged by relay contacts once the output voltage has built up. Disadvantages of this solution are the space required for the initial charging device, the lack of reliability, and the limited life of mechanical contacts.
The object of the invention is thus to provide an initial charging apparatus and a method and an apparatus for controlling it, which initial charging apparatus is distinguished by a small space requirement and has no mechanical contacts.
The circuit described in EP 94 120 245.9-1242 (A 2612/93) for a three-point pulsed rectifier system has, in each phase, in each case one power transistor which is connected via the output terminals of a single-phase diode bridge. The first AC voltage input terminal of the diode bridge is connected via a series inductance to a mains voltage terminal, and the second AC voltage input terminal is connected directly to the capacitive center point of the output voltage.
Furthermore, that output terminal of the diode bridge, which is positive with respect to the polarity of the output voltage, is connected via a freewheeling diode to the positive output voltage rail, and the negative output voltage rail is connected via a further freewheeling diode to the negative output voltage terminal of the diode bridge.
The mains current flow which is produced by the input inductance, is generally sinusoidal and is in phase with the respective mains voltage which takes place continuously during one mains voltage or input current half-cycle via one of the input-side diodes of the single-phase bridge. The basic idea of the invention is now, in each phase, to replace the input diode, which is connected to the positive output terminal of the single-phase diode bridge, by a thyristor of the same polarity and to arrange, in parallel with this thyristor, a series circuit comprising a series resistor and a diode pointing in the current flow direction of the thyristor.
Since, owing to the total current which is forced to zero in the phases, a phase current in each case always has a positive mathematical sign, that is to say physically flows into the mains input terminal of a single-phase diode bridge, the inrush current is limited in each case by one series resistor when the rectifier system is started up. After the end of the initial charging process or after reaching a minimum output voltage value which is dependent on the mains voltage, the thyristors can be triggered by a higher-level control apparatus, and the phase initial charging devices can be bridged.
A major advantage of this apparatus, in addition to avoiding mechanical contact, is the capability to integrate the thyristors, and possibly also the initial charging diodes in modules which combine the power semiconductors of one phase, thus allowing the required heat-sink area and the external circuit complexity as well as the costs of the initial charging device to be minimized.
A method according to the invention for actuating the initial charging thyristors is described herein. In this case, after the end of the initial charging of the output circuit (as described above, derived from a voltage or time condition), the thyristor is triggered only within the positive mains voltage half-cycle. This is required for carrying the continuous input current with the triggering taking place shortly before the zero crossing of the phase voltage to positive values and thus being maintained until shortly after the zero crossing to negative values in order to ensure a continuous current flow taking account of the switching-frequency ripple of the input current.
Advantages over continuous triggering include the greater resistance of the thyristor to steep reverse-voltage changes occurring within the negative voltage half-cycle. Furthermore, if there is no gate current, the reverse currents of the thyristors and thus the reverse-flow losses are considerably reduced.
The reduction in the reverse-flow current is also a major factor with regard to the capability of the system to be operated in standby. A high reverse current results in the output capacitor that is connected between the output voltage center point and the negative output voltage rail which is discharged via the diode that faces from the output voltage center point to the cathode of the initial charging thyristor and thus interferes with the symmetry of the output voltages, which is essential for operation of the system, without any possibility of correction.
An advantageous apparatus for providing the actuation method is described herein.
The voltage supply for the actuation apparatus is in this case provided via a floating DC voltage source which is connected to the negative potential (also referred to in the following text as the reference-ground potential or ground) on the thyristor cathode. In each phase, this DC voltage source is connected by an electronically operated switch-on contact to the signal-processing section of the initial-charging device, which is formed by a voltage divider, a reference voltage generator and a comparator whose output is connected to the gate of the initial charging thyristor.
The reference voltage is tapped off, for example, from the cathode of a zener diode which is connected via a series resistor to the positive supply voltage, and is connected to the negative input of the comparator, with the anode of the zener diode being connected to ground.
Furthermore, a resistive voltage divider is arranged between the positive supply voltage rail and the mains-side input of the bridge path, and its tap is connected to the positive input of the comparator and is connected via a smoothing capacitor to ground and via a diode to the cathode of the zener diode in order to limit the voltage occurring at the positive comparator input.
Once the initial charging of the output circuit of the pulsed rectifier system has been completed, the actuation apparatus is activated by means of a higher-level control and monitoring device by closing the switch-on contact, thus applying the reference voltage to the negative comparator input.
If actuation of the power transistor is now enabled and the mean value with respect to time of the voltage (which has a discontinuous profile at the switching frequency) at the input of the bridge path rises corresponding to the mains voltage profile, ignoring the voltage drop at the fundamental frequency across the series inductance, the respective phase-voltage can be formed at the input with the exception of zero components, which do not influence the zero crossing, at the input of the bridge path and with the voltage divider between the positive supply voltage and the bridge path input being dimensioned appropriately. The voltage at the positive comparator input will exceed the reference voltage and thus initiate switching of the comparator output and/or actuation of the thyristor.
In a corresponding way, once the mean bridge path input voltage passes through 0, the actuation of the thyristor is suppressed once again, that is to say the thyristor is triggered in a corresponding manner only within that mains voltage half-cycle in which it is required in order to carry the input current.